system-arm: Move display node into a shared DTS file

system-arm: Move display node into a shared DTS file

armv7, armv8, armv8_big_little DTS files are reusing the same
encoder node; moreover those should really be cpu specific files.

For these reasons, and to make it possible to craft a final DTS
without defining a display phandle, we move the shared code into
a display DTS include file

Change-Id: I4f756807292e492a743bb9ab9ec511011125a436
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38795
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>